1. Field of the Invention
The present invention generally relates to a valve timing control for a marine drive, and more particularly to an improved valve timing control for a marine drive that includes a variable valve timing mechanism.
2. Description of the Related Art
A marine drive such as an outboard motor has a marine propulsion device powered by an engine. The propulsion device typically is a propeller and is submerged when an associated watercraft rests on a body of water. The outboard motor can employ either a two-cycle engine or a four-cycle engine. Recently, however, many outboard motors are offered with four-cycle engines because they provide better emissions control.
Typically, a four-cycle engine includes one or more intake and exhaust valve moving between an open position and a closed position. One or more camshafts can be provided to actuate the valves in a timed manner. When the intake valves are open, air is introduced into combustion chambers of the engine through the intake ports. When the exhaust valves open, exhaust gases are discharged from the combustion chambers through the exhaust ports.
The engine typically includes one or more throttle valves to regulate an amount of air delivered to the combustion chambers. The throttle valves can be operated by the operator with a throttle lever. In general, the more the throttle valves open, the more power is output from the engine.
The engine can include a hydraulically operated variable valve timing (VVT) mechanism that can change the ing and closing timings of the respective valves by changing an angular position of the camshaft or camshafts. A control device such as, for example, an electronic control unit (ECU) is used to control the VVT mechanism under various control strategies. For instance, the ECU controls the VVT mechanism either to set the valve timing at a fully advanced position for relatively high engine speeds to ensure high charging efficiency and high performance of the engine, or to set the valve timing at a fully retarded position for relatively low engine speeds to ensure high combustion efficiency, fuel economy and good emission control. Otherwise, the ECU controls the VVT mechanism to set the valve timing at a position between the fully advanced position and the fully retarded position in response to a running condition of the engine.
The ECU can control the VVT mechanism to provide any position between the fully advanced position and the fully retarded position in response to, for example, a throttle valve position and an engine speed. Because throttle valves typically are cable-actuated the throttle position changes almost simultaneously with throttle lever movements. However, the engine speed changes after a certain delay following a charge in the throttle degree position.